1. Field of the Invention
This invention relates generally to devices for drawing cable or rope, and more particularly to power hoists for raising and lowering scaffolds and the like along a cable or a wire rope.
2. Prior art
(a) General history: The basic patent in this area is U.S. Pat. No. 3,231,240, which issued in 1966 to Ichinosuke Naito. It describes the concepts of using a chain-like member to press the cable into a peripheral groove in a driven sheave to obtain traction between the cable and the sheave, and applying the weight of the load to tension the chain-like member so that the traction on the cable is proportional to the load. The Naito patent was directed to stretching or moving the cable through the apparatus, with the tacit assumption that the apparatus was stationary.
Naito's invention, essentially the first generation of devices of its kind, made it possible to reliably tension and move cable of any length, without need of a drum on which to wind and store the cable. The improvement in bulk and weight were significant.
Many applications of this basic invention have since been developed. One line of such applications is the development of hoists for the movable scaffolds used in constructing and maintaining many kinds of structures, such as ships, bridges, dams and--most frequently--the exteriors of tall buildings. Such a scaffold moves up and down along cables or wire ropes that are anchored to the top of the particular structure. Generally unchanged are the basic principles of drawing the cable through the apparatus and pressing the cable into a peripheral sheave groove proportionally to the load. Here, however, what is stationary is the cable, and what moves is the apparatus--the hoist mechanism, a motor to power it, and of course the scaffold and its cargo and crew.
Among the patents directed to application of the Naito principles to scaffold hoists are U.S. Pat. No. 3,944,185, which issued in 1976 to Michael Evans, and U.S. Pat. No. 4,139,178, which issued in 1979 to Wilburn Hippach. The Evans patent introduced several features aimed at this specialized application--in particular, a secondary sheave used for at least three distinct purposes. One of these purposes was to tension the traction chain from both ends rather than only one end. Another purpose was to act as the driving end of a gear train to develop a mechanical output signal indicative of cable speed, for use in an automatic overspeed braking system. Yet another purpose was to help guide the unloaded end of the cable out of the apparatus.
Hippach provided further refinements directed to the reliability (particularly reliability under extreme operating conditions) and the convenience in use of the apparatus. The Hippach patent describes subtle features of the overspeed-brake gear train, designed to ensure smooth operation of the mechanism under extremely high accelerations; and also describes what could be called spring-preloading of the secondary sheave, to facilitate automatic reeving or "threading" of the cable through the apparatus.
Thus these patents may be regarded as the second generation of cable-drawing equipment developments, in the scaffold-hoist field. They were directed to producing optimum performance in terms of reliability and convenience.
Modern users of industrial equipment, however, demand more than this. The present age is extremely conscious of the usage of energy, particularly nonrenewable energy sources. The modern age is also extremely conscious of the usage of materials, particularly metals.
It has therefore become a matter of paramount concern to all manufacturers, and certainly to manufacturers of scaffold hoists, that apparatus be efficient in terms of energy usage, and that its construction use no more material than need be--while remaining just as reliable and convenient as before.
(b) Hoist weight considerations: Such concerns of course render it undesirable to construct hoists that are relatively heavy. Past hoists have not been greatly overweight, of course, and they have been the state of the art.
Still, under the modern conditions outlined above they may not have been optimum, both because of the relatively large amounts of metal that must go into their construction and because of the continuing costs of hoisting their own weight--to the extent of whatever "excess" weight they may have.
(c) Multiple-cable-size considerations--efficiency: Perhaps less plain, but equally significant in terms of energy and materials efficiency, is the undesirability of making several different models of hoists for use with cables of different sizes. It has been a standard practice in the hoist industry to make either different models, or models with different modules, for use with cables of different sizes.
The use of cables of different sizes arises from the various loads which scaffolds must carry, and to some extent from variety in the local safety statutes with which users must comply, and also from the special circumstances and preferences of users. Thus it is neither possible nor particularly desirable to eliminate nonuniformity of cable sizes in use.
Yet there are many inefficiencies in the practice of manufacturing different hoists for the different cable sizes. Such inefficiencies extend through warehousing, spare-parts maintenance, billing and bookkeeping systems, and communications complexity all along the distribution chain from manufacturer to user. In addition, for a user who wishes to use cables of different sizes within his own operations, for different scaffolding purposes, the expense and inconvenience of having to own more than one hoist model or module are particularly salient.
(d) Multiple-cable-size considerations--reliability of performance: For such a user the problems arising from ownership of different hoist equipment can also pose a procedural problem: constant vigilance must be exercised when personnel have been using one cable size and switch to another, to be sure that the right hoist has been selected for use with that other cable size--or, even more insidiously, to be sure that the right cable-size-dependent module has been selected.
Interestingly enough, the area in which cable-size-dependent modules have most prominently been introduced is the area of overspeed brakes. The practice of providing different brake components for use with different cables is particularly unfortunate in view of the fact that overspeed brakes, by their nature, are not actually placed into service until an overspeed condition (i.e., emergency) occurs.
Generally speaking, if a hoist being used with a cable of small diameter has attached to it a brake designed for use with a cable of large diameter, the hoist will operate to drive the scaffold up and down the cable; there is nothing inherent in the mismatch, but only the user's watchfulness, to prevent the user from proceeding--but generally if an emergency arises the brake will not work at all. In some cases the same problem is present when using a large-diameter cable and a brake designed for a small-diameter cable.
(e) Power-transmission systems: In another field, the field of mechanical power-transmission devices, certain basic developments have arisen which have never been used in hoists. U.S. Pat. No. 4,194,415, which issued in 1980 to Frank Kennington and Panayotis Dimitracopoulos, describes a "quadrant drive" system.
This system provides mechanical motion transmission with a large mechanical advantage, using extremely lightweight construction by comparison with conventional gear trains. Yet the quadrant drive has all the load-bearing and torque-transmitting capability of the heavier conventional gearing.
The quadrant drive accomplishes this by using an eccentric gear-like input drive wheel that drives a multiplicity of small drive pins at the periphery of the wheel. The drive pins are constrained to follow an ovoid path, about half of which path follows the teeth on the eccentric wheel (so that the drive pins are engaged with the teeth on the eccentric wheel), and the other half of which path is spaced away from those gears. The pins are simultaneously constrained to move in radial slots--or to bear against other drive-pin-engaging elements--in another wheel or plate.
Some manufacturers have introduced devices related to the quadrant drive, such as the Graham Company's "circulute reducer". The principal developer of the quadrant drive has been the Swiss firm Plummettaz S. A.
In some quadrant-drive devices the pins are always engaged with this second plate, and in others they are engaged with this second plate at least whenever they are on that part of their path which follows the teeth on the eccentric wheel. Moreover, as already mentioned, they are about half the time engaged with the eccentric wheel; thus the driving load is at all times borne by about half the pins, and by about half the teeth of the eccentric drive wheel, and by about half the radial slots (or other drive-pin-engaging elements) of the driven plate--rather than by only two or three gear teeth.
The result is a great improvement in torque-to-weight ratio, since a much more lightweight construction may be used to obtain the same load-bearing and torque-transmitting capability.
By their nature, however, quadrant (or circulute) drives are relatively bulky, and somewhat cumbersome to use in portable equipment--particularly equipment, such as scaffold hoists, in which space is at a distinct premium. If others in the hoist industry have taken note of the quadrant drive (and we have no indication that such an event has occurred) perhaps they may have been deterred by the seeming awkwardness of mating the lightweight--but somewhat cumbersome--quadrant drive to the traditionally and ideally compact scaffold hoist.
At least two other complications tend to teach away from the concept of using quadrant or circulute drives in scaffold hoists. First, such drives provide a mechanical advantage ratio that is--while relatively high for a single stage--somewhat limited in comparison with an entire conventional gear train. Typical single-stage commercial units have ratios no higher than sixty or seventy to one. Of course two-stage units (two quadrant drives connected in series) produce extremely high reduction ratios, as large as the square of the ratio produced by highest-ratio single-stage units--some 5000 to one. Two-stage units, however, would be all the more bulky and awkward, and for scaffold-hoist applications would lose a great deal of the torque-to-weight ratio advantage of the single-stage units.
Second, the mechanical advantage of a quadrant drive is not readily modified; that is to say, the drive has a mechanical advantage that is quite firmly built into the device. (In a conventional gearbox, by contrast, changing two spur gears at one end of the train or the other can provide desirable refinements of the overall reduction for particular applications.) Thus, even if quadrant drives were available with high enough single-stage reductions for scaffold hoists, their use in such applications would require hoist manufacturers (and some users) to stock and service a variety of drives with various reductions, to satisfy the gearing requirements of different hoist applications.
(f) Summary: The foregoing comments show that there has been a need in the scaffold-hoist industry for a third generation of hoists, substantially lighter in weight than those of the second generation but just as convenient and reliable, and capable of accommodating any of several different cable sizes without change of hoist--or hoist components. This need arises from considerations of energy and materials efficiency, and efficiency in general, and also from considerations of reliability in use.
These comments also show that the quadrant or circulute drive has some tantalizing benefits for the scaffold-hoist industry, but that certain inherent characteristics and certain commercial characteristics of the quadrant drive have seemed to make it incompatible with the requirements of such hoists.